PROJECT SUMMARY While common, evolutionarily conserved themes of early neuronal patterning in the developing embryo have emerged over the years, there are, as of now, no common organizational themes for how neuron acquire and maintain their terminally differentiated state. We propose to test here the hypothesis that each neuron class of the nematode C.elegans is uniquely defined ? and functionally specified ? by a neuron class-specific combination of transcription factors, with at least one of the transcription factors being a homeodomain-type transcription factor. The finding that homeodomain transcription factors control the identity of all neurons in a simple nervous system would have wide- ranging implications for our understanding of evolution of the nervous system and may provide means to classify (and then functionally study) neuron classes in mammalian nervous systems.